Optimize FrontierChallenge site for mobile

[Chord-Chen-30]

Summary

• mark the canonical GitHub Pages URL and source directory
• optimize navigation, resource links, leaderboard scrolling, case cards, detail pages, and footer for common iPhone and Android widths
• prevent page-level horizontal overflow while preserving touch-friendly table scrolling
• refresh the stylesheet cache key across the static pages

Verification

• checked all 12 static pages at 360px and 430px widths
• checked the homepage at 375x667, 390x844, 430x932, 360x800, and 412x915
• confirmed no page-level horizontal overflow or missing images
• confirmed all 13 leaderboard rows render and long model names remain within their cells

The former standalone FrontierChallenge-site repository is not modified or used as the deployment target.

[zhanghanduo]

LGTM